Iambic pentameter is a line of verse drama with five groups of syllables where the first is unstressed and the second is stressed. Shakespeare's Sonnet 18 compares the subject's beauty to summer and says their beauty will not fade over time like summer does. It also references two of Shakespeare's most famous quotes from Hamlet and Romeo and Juliet which vary from the typical iambic pentameter structure.

1. Iambic Pentameter
A line in verse drama that expresses a particular
rhythm.
Iambic = Type of “foot” (groups of syllables) that
describes an unstressed syllable followed by a
stressed syllable.
Pentameter = Indicates the lines has five “feet”

2. Sonnet #18
Shall I compare thee to a Summer’s day?
Thou art more lovely and more temperate:
Rough winds do shake the darling buds of May,
And Summer’s lease hath all to short a date:
Sometime too hot the eye of heaven shines,
And oft’ is his gold complexion dimm’d;
And every fair from fair sometime declines,
By chance or natures changing course untrimm’d:
But thy eternal Summer shall not fade
Nor lose possession of that fair thou owest;
Nor shall Death brag thou wanderest in his shade,
When in eternal lines to time thou growest:
So long as men can breathe, or eyes can see,
So long lives this, and this gives life to thee.

3. Shakespeare’s poetic variations
--Hamlet
“To be or not to be, that is the question”
Romeo and Juliet
“It is my lady; O, it is my love!
O that she knew she were!”